DOC dataset doc add return and improve doc #3315

Fixes #3315 
What do you think @arjoly ?
What's wrong with not indenting? I don't mind:

```
bunch : Bunch object with the attributes as follows
bunch.data: sparse matrix, shape [n_samples, n_features]
bunch.target: array, shape [n_samples]
bunch.target_names: list, length [n_classes]
```